The present invention relates to the diagnostic imaging arts. It finds particular application in conjunction with open MRI systems when imaging the ligaments of the knee and will be described with particular reference thereto. It will be appreciated, however, that the present invention is also useful in conjunction with X-ray, CT, and other imaging modalities.
In magnetic resonance imaging, a uniform main magnetic field is created through an examination region in which a subject to be examined is disposed. With open magnetic resonance systems, the main magnetic field is generated vertically, between upper and lower pole pieces. A series of radio frequency (RF) pulses and magnetic field gradients are applied to the examination region to excite and manipulate magnetic resonances. Gradient magnetic fields are conventionally applied to encode spatial position and other information in the excited resonance.
When a patient injures his knee, doctors are often interested in the damage the ligaments of the knee have sustained. Unfortunately, the ligaments of the body fit snugly against bones, and as a result, are hard to image using non-invasive imaging techniques. In order to achieve acceptable contrast between the ligament to be imaged and the surrounding tissue, the bones of the knee are moved away from the ligaments.
When using magnetic resonance imaging, the subject remains still during the examination. The main magnetic field is encoded with three dimensional gradient fields that precisely map the imaging area. It is important to have the same region of the subject in the same imaging area throughout the whole imaging process. If the subject moves during imaging, then the parts of the body being imaged will correspond to the wrong spacial position and the resultant image will be an incorrect, double-exposure or blurred representation of the anatomy of the subject. Consequently, the region of the patient being imaged should remain still during the process.
Magnetic resonance compatible fixtures of nylon or aluminum and foam pads have been used to hold a patient""s knee at a selected angle during imaging. However, such fixtures and pads only limit or prohibit movement. They are not designed to apply anterior or posterior loading.
A device attributed to M. L. Stone et al as described in the American Journal of Sports Medicine, 13:401-407, 1985 Was designed to provide loading on the crucial ligaments. However, the device was not MRI compatible. The patient""s knee and foot were supported by an anvil or pivot block and a foot stop block, respectively. The arthometer was strapped to the calf with a tibial sensor pressed against the tibia and a patella sensor pressed against an end of the patella. The operator gripped a force handle on the top of the arthometer with one hand and the patella sensor in the other. In one mode, the operator pulled up on the handle and pushed down on the patella sensor until a gauge or audio signal indicated 7 kg or 9 kg of force. The arthometer was unable to increase the forces on the knee in small, accurate steps. Moreover, it was unsuited to imaging the knee at a series of incremental loading steps.
The present invention provides a new and improved method and apparatus that overcomes the above referenced problems and others.
In accordance with one aspect of the present invention, an MRI scanner generates a main magnetic field through an imaging region where a subject is disposed. The scanner encodes dipoles in the subject by superimposing gradient fields on the main magnetic field. Magnetic resonance in the selected dipoles is excited and detected by radio-frequency coils and processed into an image representation of the selected area of the subject.
In accordance with another aspect of the present invention, a ligament loading device is disposed in the imaging region which secures the leg of a patient during imaging. The device has the ability to apply anterior/posterior pressure to the patient""s leg for the purpose of separating the bones of the knee to make the ligaments of the knee more visible for imaging.
In accordance with a more limited aspect of the present invention, the ligament loading device applies pressure to the patient""s leg through the use of operator controlled pneumatic sacs which inflate and cause movable plates to put pressure on the patient""s leg. In this manner the operator can feel how much pressure is being put on the leg.
In accordance with another aspect present invention, a method of magnetic resonance imaging is provided where the leg of a patient is disposed in a ligament loading device, a main magnetic field is generated in the region, pressure is applied to the upper and lower portion of the leg in opposing directions, magnetic resonance is excited in the region, the magnetic resonance signals are received, demodulated, and processed into an image representation of the patient""s knee.
In accordance with a more limited aspect of the present invention, multiple such images are taken at varying pressures and animated in sequence to yield a cinematographic representation of the patient""s knee.
One advantage of the present invention is that it is MRI compatible.
Another advantage of the present invention is that it firmly fixes the knee for imaging.
Another advantage of the present invention is that the amount of force applied is easily limited to safe levels.
Another advantage of the present invention is that it allows the force applied to be read numerically.
Another advantage of the present invention is that it allows the physician to physically feel the amount of force being applied.
Another advantage of the present invention is that it allows for diagnostic imaging of the knee injury without radiation.
Another advantage of the present invention is that it allows for multi-frame imaging over a range of pressures resulting in a cinematographic representation.
Yet another advantage of the present invention is that it is of simple design and low maintenance.
Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.